How to hide data in look-up field - using criteria hold in main form from previous records?

I am working on an application to book services and assign them to employees (employees are selected using a lookup field).

* Booking form has : start date, end date, selected service, select employee --> drop-down lookup field (linking to add/edit employees) Once the worker is selected a decision box: "mark worker as busy" is ticked.
==> The idea is when the form is submitted, the employee is marked as busy for the period " start date - end date"

* Add/edit employees form is used to hold: Name, email, phone number of the employees.

I am trying to set the booking form so that the lookup field does not show the employees marked as busy for selected start and end time.

Any idea on how to do this? Is it even possible?

I have looked a dynamic drop-down, but the criteria has to be in the same database of the lookup field.

I am thinking about either:
- a form action - on load, but I am not sure how this can work with the lookup field. (can the lookup field be set as a list?)
- create a subform based on "booking form" in add/edit employees with start date / end date/ mark as busy to then set the criteria for dynamic drop down.

Many thanks in advance for your advises & suggestions,

Kind regards,
Claire